Acetylene-terminated polyimide adhesives

In a research programme to develop an addition polyimide adhesive that is easily processed and exhibits good retention of strength after long term ageing, three different acetylene-terminated polyimide adhesives systems were prepared and characterized. The most promising system, a polyimidesulphone, was selected for a long term ageing study. The results indicate that this polyimidesulphone has improved strength retention over LARC-13 after ageing for extended time at high temperatures. In addition, it exhibited surprising adhesive strength for a short time at 593°C. The synthesis and characterization of these adhesives are discussed.     

Polyetherimide Adhesive

High-temperature adhesives which can be adhered at adhesive temperatures lower than those of conventional polyimide adhesives were investigated. Polyetherimide (PEI), developed by General Electric Co., is one such promising low curing temperature adhesive because it melts at temperatures lower than those used for conventional polyimides. Lap shear adhesive strength was investigated in a 75 μm-thick PEI film using steel test pieces. 350 kgf/cm² was achieved after curing for 1 hour at 270°C and 150 kgf/cm² was achieved at the test temperature of 200°C. PEI adhesive dissolved by N, N-dimethylformamide exhibited a high adhesive strength of 240 kgf/cm² after curing for 2 hours at 200°C. In addition, it was found that PEI could be used at much lower adhesive pressures than those of conventional polyimide adhesives.     

Comparative study of zein- and gluten-based wood adhesives containing cellulose nanofibers and crosslinking agent for improved bond strength

Many of the currently used wood adhesives contain chemicals that are harmful to human health and the environment. Increasing environmental and human health concerns have made the development of safe biobased adhesives a priority. In this study, two plant proteins, i.e., zein and wheat gluten, were used to develop wood adhesives and their performance was compared through simple lap shear tests and plywood flexural/internal bond tests in dry and wet conditions. To increase their bond strength, cellulose nanofibers were added to create nanocomposite adhesives and glutaraldehyde was also used to crosslink the proteins. Single-lap shear test was performed to measure the bond strength of different adhesive formulations and determine the optimal formulations and processing conditions. Fractured bond surfaces were studied using optical observation and scanning electron microscopy to determine bond failure mechanisms. Thermal and chemical properties of the adhesives were evaluated using thermogravimetric analysis and Fourier transform infrared spectroscopy, respectively. The bond strength of both zein and gluten adhesives was significantly increased by the addition of the cellulose nanofibers and/or glutaraldehyde, although the two adhesives responded differently to the two reinforcement materials due to the different solvents used to prepare the adhesives. The bond failure mode changed from cohesive failure of the adhesive to structural failure of the adherent for the gluten adhesive containing CNFs and glutaraldehyde. Potential zein and gluten adhesive formulations were used to produce plywood samples and their performance was assessed under different conditions. The formulations with industrial potential were discovered through this study.     

Benchmark tests on adhesive strengths in butt, single and double lap joints and double-cantilever beams

The RC99 committee of the Japan Society for Mechanical Engineers conducted the benchmark tests on strengths of adhesive joints using different testing methods. The effects of joint configuration, loading mode, adherend yield strength and so on, on the strength and data scatter were investigated using two typical epoxy adhesives. The strengths obtained by various tests were compared with each other. The relationships among strengths of butt, single lap and double lap joints and fracture toughness were given. Thirteen member institutes of the committee participated in this project. The benchmark results allow us to recognize that the joint strengths are strongly affected by the curing process. The key to obtaining the appropriate joint strength, is precise temperature control inside the adhesive layer for curing. Toughened adhesives do not always give higher joint strengths than untoughened adhesives. The yield strength of adherends much affects the observed lap joint strength of adhesives.     

Flexibilized Copolyimide Adhesives

Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI.

Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours.

The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)].

These flexible thermoplastic copolyimides show considerable potential as adhesives based on this study and because of the ease of preparation with low cost, commercially available materials.     

Effect of Bondline Thickness on Mixed-Mode Debonding of Adhesive Joints to Electroprimed Steel Surfaces

Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for techniques to assess adhesive joint strength, particularly along bondline interfaces where compliant adhesives contact more rigid metallic surfaces. The present study investigates the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel adherend surfaces. Three bondline thicknesses, representative of structural automotive joints, were evaluated for unprimed and primed bondlines. Experimental results for static load versus debond extension were input to finite element analyses for computing debond parameters (strain energy release rates). The debonds always initiated at a through-the-thickness location that had the greatest peel component of strain energy release rate. The total strain energy release rate values correlated well with trends in joint strength as a function of bondline thickness.     

The Effect of Moisture on Polyurethane Adhesives

Polyurethane adhesives predominate for bonding shoe soles but the moisture resistance of the adhesive polymer has been questioned. A typical polymer was hydrolytically degraded at 120°C for up to 29 hours and at 20, 30 or 37°C for up to one year and changes in physical and bonding properties monitored. Considerable degradation was necessary before bonding was seriously impaired due to cohesive failure of the adhesive, although heat resistance declined more rapidly. Joints made with undegraded adhesive on exposure to moisture showed rapid loss of strength and cohesive failure at 60 or 70°C, but little loss of strength and failure at or near the interface at 30 or 40°C.     

Structural Adhesive Bonds to Primers Electrodeposited on Steel

Adhesive bond strength and durability were investigated for steel substrates which had been cathodically electroprimed before bonding. Lap shear and torsional impact strengths of two model epoxy adhesives were evaluated. Very poor strengths and durability were found for one adhesive, which was cured with a mixture of three amine curing agents. Scanning electron microscopy and analysis of primer susceptibility to interaction with the curing agents suggested that, for the high concentrations of curing agent in the amine-cured adhesive, chemical and physical degradation of the primer occurred during cure at elevated temperature.

For the second adhesive, which was cured with a single imidazole catalyst, excellent strength and durability were obtained, with no evidence of primer degradation. Surprisingly, for this adhesive, strengths to primed steel were up to 88% higher than to cleaned (i.e., degreased) bare steel. The concurrent improvements in environmental durability over bare steel, as assessed by water immersion and salt spray accelerated exposures, were attributed to the more favourable surface energetics of the adhesive/primer interface.     

Ageing of Adhesive Bonds with Various Surface Treatments, Part 2: Acrylic Modified Amine-Cured Epoxy Resin

Ageing of adhesive-bonded aluminium-diglycidyl ether of bisphenol A based aliphatic amine-cured acrylic modified epoxy resin reinforced with fused silica exposed to 100% humidity and cycled between 42-48-42 °C every hour is reported. Moisture uptake was followed using broadband dielectric measurements. It was found that the adhesive absorbs moisture very rapidly, the adhesive undergoes significant swelling, and there is a consequent reduction in strength consistent with plasticization of the matrix. The dielectric measurements parallel closely the changes occurring in the adhesive bond line and provide a nondestructive method for monitoring the changes in bond strength. The changes that occur are dominated by the effects of moisture on the adhesive, various pretreatments have little effect on the ageing process, and interfacial failure in this case does not make a significant contribution to the failure of these joints.     

The preparation and performance of high-temperature adhesives for graphite bonding

Two kinds of high-temperature adhesives (HTAs) were prepared. One was composed of phenol-formaldehyde (PF) resin and boron carbide (PF+B₄C), the other was composed of PF resin, B₄C and fumed silica (PF+B₄C+SiO₂). Graphite materials were bonded by the above adhesives and heat-treated at temperatures ranging from 200 to 1500 °C. The joining strength was tested at room temperature. The results show that the graphite joints exhibit satisfactory bonding strength and that ceramics fillers show a marked property modification effect. The strength of graphite joints bonded by PF+B₄C and PF+B₄C+SiO₂ adhesive and treated at 1500 °C are 9.3 and 17.1 MPa, respectively. The property modification mechanism of ceramics fillers is also discussed in this paper. A strong chemical bonding force is introduced at the bonding interface and the volume shrinkage is restrained, which can be responsible for the good adhesive properties of HTAs for graphite bonding.     

Characterization of adhesive ageing using mechanical and electrochemical inspection

Two methacrylate adhesives, commonly used in railway applications, were tested before, during and after accelerated humid ageing. First, bulk tensile samples were fabricated in order to perform mechanical characterization before ageing. Then, the samples were placed in a humid environment and the influence of water on the Young's modulus and the tensile strength was determined. In parallel, Electrochemical Impedance Spectroscopy (EIS) measurements were carried out with methacrylate-adhesive-coated aluminum panels in order to extract parameters related to the porosity and adhesion of adhesives which are both dependent on water penetration. Finally, the mechanical characterization of adhesively-bonded lap joints with aluminum substrates was performed before and after accelerated humid ageing. All these experiments allowed to distinguish the two adhesives tested. It was found that the bulk adhesive samples which presented the best mechanical properties before ageing did not guarantee a good bonded-joint behavior after ageing. More importantly, the combination of EIS with mechanical tests allows to significantly reduce very much the number of experiments and, thus, the cost of industrial validation tests.     

Rubber solution adhesives for wood-to-wood bonding

Rubber solutions were prepared and used for bonding wood pieces. The effect of the variation of chlorinated natural rubber (CNR) and phenolformaldehyde (PF) resin in the adhesive solutions on lap shear strength was determined. Natural rubber and neoprene-based adhesive solutions were compared for their lap shear strength. The storage stability of the adhesive prepared was determined. The change in lap shear strength before and after being placed in cold water, hot water, acid, and alkali was tested. The bonding character of these adhesives was compared with different commercially available solution adhesives. The room-temperature aging resistance of wood joints was also determined. In all the studies, the adhesive prepared in the laboratory was found to be superior compared to the commercial adhesives. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1185–1189, 1998     

Effect of Adhesive Type and Thickness on the Lap Shear Strength

The effect of the adhesive thickness on the bond strength of single-lap adhesive joints is still not perfectly understood. The classical elastic analyses predict that the strength increases with the adhesive thickness, whereas experimental results show the opposite. Various theories have been proposed to explain this discrepancy, but more experimental tests are necessary to understand all the variables.

The objective of the present study was to assess the effect of the adhesive thickness on the strength of single-lap joints for different kinds of adhesives. Three different adhesives were selected and tested in bulk. The strain to failure in tension ranged from 1.3% for the most brittle adhesive to 44% for the most ductile adhesive. The adherend selected was a high-strength steel to keep the adherends in the elastic range and simplify the analysis. Three thicknesses were studied for each adhesive: 0.2, 0.5, and 1 mm.

A statistical analysis of the experimental results shows that the lap shear strength increases as the bondline gets thinner and the adhesive gets tougher.     

Preparation and Properties of Aramid Copolymers Derived from 3,4'-ODA, 4,4'-ODA, IPC and TPC

Two series of wholly aromatic copolyamides (aramid copolymers) derived from (3-aminophenyl) (4-aminophenyl) ether (3,4'-ODA), bis(4-aminophenyl)ether(4,4'-ODA), isophthaloyl chloride (IPC), and terephthaloyl chloride (TPC), having inherent viscosity of 0.63-1.41 dL·g^-1 were synthesized. The thermal and mechanical properties of the cast films were investigated for application as hot-melt adhesives. When the aramid 341 composed of 3,4'-ODA and IPC was used, the value of adhesive joint strength at 20°C was 8.4 MPa and this value was maintained even at 230°C. And when 20 mol % of 3,4'-ODA were replaced by 4,4'-ODA, the value of adhesive joint strength at 20°C was a maximum, 16 MPa.     

Bacterially derived biopolymers as wood adhesives

The wood adhesive market is very large and problems due to volatile organic compounds and toxic chemicals in many adhesives and their production are significant. In addition, most of the adhesives are derived from depleting petrochemical resources. An environmentally friendly wood adhesive based on renewable resources and produced by microbial fermentation has been explored. Using the shear block test method, a microbially produced polysaccharide has been tested and the effects of wood type, humidity, set time, partial acetylation, and surface wetting agents were determined. Shear strength of the microbial polysaccharide adhesive was compared to that of a commercial wood adhesive and other polysaccharides. Shear strengths of up to 20 MPa (3000 psi) for bonding maple have been obtained at 53% relative humidity and 22°C.     

Experimental study of the effect of microspheres and milled glass in the adhesive on the mechanical adhesion of single lap joints

This article describes several experiments conducted on single lap joints (SLJ) subjected to tensile mechanical loads. Two epoxy adhesives, with slow and fast curing, were used, with a weight of 0%, 3%, and 10% of glass microspheres and milled glass particles, respectively. The adherends used in the construction of the specimens were fiber-reinforced polymers. The types of failures produced in the SLJ specimens were classified according to ASTM standards. The results of the experimental tests on the SLJ with fast-curing epoxy adhesive showed that the use of milled glass and glass microspheres improved the strength of the joint compared with the neat fast-curing epoxy adhesive. As for the experimental test on the joint with slow-curing epoxy adhesive, the results showed that the use of milled glass and glass microspheres decreased its strength when using different additive concentrations compared with the neat slow-curing epoxy adhesive.     

The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive

One parameter that influences the adhesively bonded joints performance is the adhesive layer thickness. Hence, its effect has to be investigated experimentally and should be taken into consideration in the design of adhesive joints. Most of the results from literature are for typical structural epoxy adhesives which are generally formulated to perform in thin sections. However, polyurethane adhesives are designed to perform in thicker sections and might have a different behavior as a function of adhesive thickness. In this study, the effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive was investigated. The mode I fracture toughness of the adhesive was measured using double-cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 0.2 to 2 mm. In addition, single lap joints (SLJs) were fabricated and tested to assess the influence of adhesive thickness on the lap-shear strength of the adhesive. An increasing fracture toughness with increasing adhesive thickness was found. The lap-shear strength decreases as the adhesive layer gets thicker, but in contrast to joints with brittle adhesives the decrease trend was less pronounced.     

Adhesion Tensile Testing of Environmentally Exposed Ti-6Al-4V Adherends

The structural and bonding properties of Ti-6Al-4V adherends, prepared by chromic acid anodization (CAA), were studied as a function of exposure in high-temperature environments such as vacuum, air, boiling and pressurized water, and steam. Subsequent to the environmental exposure, bonds were produced and the adhesive tensile strengths measured. Long-term exposure to high temperature, dry environments did not cause structural changes to the adherend oxide but did result in poor bond strength. The failure mode in these cases was within the oxide, which was apparently weakened by the exposure. The water-and steam-exposed oxides underwent a transition from amorphous to crystalline TiO₂ (with an accompanying change in oxide morphology); however, bond strength was maintained for moderate exposures at T ≤ 250°C. For exposure at T = 300°C, the bond strength was degraded severely. The latter result can be explained by a lack of porosity in the transformed oxide. SEM and XPS measurements were made on debonded surfaces to determine the loci of failure.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service

Room Temperature curing compositions of epoxy resins with high temperature service capability (95-120°C) were formulated and evaluated. The compositions were based on selected high functionality atomatic epoxy polymers and multicomponent poly amine curing agent systems. Toughening was achieved by addition of a rubbery phase either by prereaction of the epoxy resin with carboxyl terminated (CTBN) or by amine terminated (ATBN) poly butadiene acrylonitrile. The latter elastomeric component served as a part of the poly amine curing agent.

Best results were achieved with an adhesive formulation comprising tetra glycidyl-4-4'-diaminodiphenylmethane (TGDDM) and triglycidyl ether of p-aminophenol with triethylenetetramine and addition of ATBN with a felt carrier.

Lap shear strengths of aluminum/aluminum specimens primed by silane coupling agent in the order of 22 MPa at 25°C and 11 MPa at 120°C with T-Peel strengths of 1.6N/mm at 25°C and 0.52 N/mm at 120°C, were obtained.

The thermal behaviour and transitions, the chemical and mechanical properties, the microstructure and morphology of the selected adhesive formulation were studied, using DSC, Gehman, FTIR, mechanical testing and SEM analysis, respectively.

Experimental results showed that the selected compositions could develop good high temperature (120°C) properties while cured at room temperature. Furthermore, their high temperature performance compares favorably or even exceeds that of commercially available room-temperature-curing adhesive compounds, and are competitive with elevated temperature cured film adhesives.     

Bond strength and durability of universal adhesive agents with lithium disilicate ceramics: A shear bond strength study

The aim was to assess the shear bond strength (SBS) of lithium disilicate (LD) ceramic to resin composite with different universal adhesives, duration of ageing and silane. One hundred and twenty LD ceramic discs were processed, fired and etched (HF acid 5%) for 20 s (sec). All specimens were divided into 12 groups (n = 10), based on different combinations of, 3 different universal adhesives [Scotchbond (SB) Universal Adhesive, All-Bond (AB) Universal, and Futurabond U (FU)], silane and different duration of ageing [24 h and 3 months]. Composite resin cylinders (Tetric ceram) (3mm × 2 mm) were formed using bonding jig on ceramic and were light-cured. The specimens in groups 1–6 and 7–12 were stored in distilled water (37 °C) for 24 h and 3 months (thermocycling -5000 cycles 5–55 °C/30 s dwell time) before being subjected to bond strength testing respectively. Using universal testing machine shear bond test was performed at a crosshead speed of 1 mm/min. Failure modes and fracture patterns were assessed using stereomicroscope and scanning electron microscope. Analysis of variance was performed to analyze data. SBS was significantly higher with silane than without silane (p < 0.01), regardless of the type of adhesive or storage duration. Specimens tested at 24 h storage showed significantly higher (p < 0.01) SBS than specimens tested after 3-months. A comparison among different universal adhesives showed significantly distinct bond strength (p < 0.01). Optimal bonds to LD were achieved by application of silane. While ageing through storage had a negative impact on the SBS, it varied among different adhesives.